(1) Field of the Invention
The present invention relates to a ring type optical transmission system which transmits wavelength division multiplexed (WDM) light between a plurality of nodes connected in a ring, and in particular to a ring type optical transmission system comprising a plurality of optical add/drop multiplexing (OADM) nodes, and an optical device which connects thereto.
(2) Related Art
The explosive increase in the demand for data communication, particularly in the demand for internet traffic, has meant that greater capacity and longer distance transmission is required from the backbone network. Furthermore, the increasingly Wide variety of services available to users means that a network is required which realizes high reliability and flexibility and is also economical.
Optical communication networks in particular, because of their position at the core of the basic form of an information communications network, are required to provide higher speed services over wider areas, and development is proceeding towards an information society at a rapid rate. Furthermore, wavelength division multiplexing (WDM) technology is widely used as the core technology in optical transmission systems. WDM transmission is a method of simultaneously transmitting a plurality of optical signals with different wavelengths over a single optical fiber by multiplexing the signals.
In nodes where WDM transmission is performed, because a variety of processing is performed in optical path units of the optical wavelength region, control in the form of optical add/drop multiplexing (OADM) is performed in which optical signals with a specific wavelength are added or dropped, for example, without converting the optical signals to an electric signal.
FIG. 19 shows an example of the configuration of a typical ring type optical transmission system which includes OADM nodes.
In the system shown in FIG. 19, for example a plurality of OADM nodes N1 to N7 centered about a central station N0 are connected in a ring by a transmission path 100. Because the central station N0 exchanges data with each OADM node, then to enable communication with all of the OADM nodes N1 to N7, after the WDM light input from the transmission path 100 is demultiplexed into distinct wavelengths, the drop, add and through processing of optical signals of each wavelength is controlled, and the optical signals of each wavelength are multiplexed again and output to the transmission path 100. As shown in FIG. 20, for example, a different wavelength group G1 to G7 is preallocated to each OADM node N1 to N7 as the add wavelengths, and nodes which are to act as communication partners are set by selecting the drop wavelengths in the local node using a wavelength tunable filter or the like. Such a construction makes it simple to set up connection paths for example in units of hours or minutes, and consequently a network can be provided which is well suited to such services as time based path sharing (wavelength sharing) services. Furthermore, setting the same drop wavelength in each OADM node enables multicast communication in which a single transmission signal is received at a plurality of points, and broadcast communication in which a single transmission signal is received by all of the nodes. Consequently, a network can be provided which is suited to image distribution and broadcasting type services which are anticipated to grow in the future.
A known example of a specific construction which realizes the function of the central station, uses a hub node as shown for example in FIG. 21. In the configuration example of FIG. 21, WDM light input from the transmission path is input into an optical demultiplexer 101 via an optical amplifier or the like, and demultiplexed into distinct wavelengths in the optical demultiplexer 101, after which signal light of each wavelength λ1 to λn is output from each port of the optical demultiplexer 101. The signal light of each wavelength λ1 to λn is branched into through light and drop light in an optical coupler (CPL) 102 corresponding to each wavelength. The through light of each wavelength λ1 to λn is sent to an optical switch (SW) 103 corresponding to each of the wavelengths λ1 to λn, and either through light or add light is selected in each optical switch 103. The optical signals corresponding to the wavelengths λ1 to λn output from each optical switch 103 are then multiplexed again in an optical multiplexer 104, and the WDM light is output to the transmission path via an optical amplifier or the like.
The hub node in the present specification signifies a node which demultiplexes the input WDM light into individual wavelengths and performs corresponding optical signal processing on each wavelength. In the configuration example in FIG. 21, by providing optical couplers 102 and optical switches 103 to correspond with each branched wavelength, the hub node is given the function of an OADM, and acts as a central station. In addition to the example above, known constructions for such hub nodes which function as an OADM include a construction where the OADM function is realized by providing for example 2×2 optical add/drop switches to correspond with each wavelength (see for example Japanese Unexamined Patent Publication No. 2004-153307).
Furthermore, to realize an OADM node as described above, a wavelength tunable optical filter is required which is capable of selecting an optical signal with the desired wavelength from within the WDM light. An acousto-optic tunable filter (AOTF) is an example of a widely used wavelength tunable optical filter. An AOTF filters the desired wavelength by inducing changes in the refractive index of an optical waveguide using the acousto-optic effect (an effect whereby light is diffracted by acoustic waves excited in a substance or on the surface of a substance), and isolating/selecting a spectral component by rotating the polarization state of light which propagates through the optical waveguide. Because this AOTF can adjust the optical wavelength for selection over a wide range by changing the value of the frequency of a radio frequency (RF) signal applied to an electrode for exciting acoustic waves formed on the optical waveguide substrate, the AOTF is a useful optical device in the construction of an OADM node.
A configuration as shown in FIG. 22, for example, is an example of a specific configuration of an OADM node using such an AOTF (see for example Japanese Unexamined Patent Publication No. 2004-235741). In this configuration example of an OADM node, the WDM light input from the transmission path is branched into two in an optical coupler (CPL) 111, from where one portion of the WDM light is sent to a rejection/add filter 121, and the other is sent to an optical coupler (CPL) 112 and further branched into four portions. The WDM light output from each output port of the optical coupler 112 is respectively applied to a wavelength tunable filter 113 using an AOTF or the like, thereby selecting the desired wavelength and extracting the drop light. Furthermore, the add light of each wavelength output to the transmission path, after being respectively amplified to the desired level in an optical amplifier 122, is multiplexed in the optical coupler 123 (CPL) and applied to the rejection/add filter 121. In the rejection/add filter 121, to prevent the add light in the local node from recirculating on the ring network, optical signals included in the WDM light from the optical coupler 111 which have the same wavelengths as the add wavelengths in the local node are terminated, and the remaining through light is multiplexed with the add light from the optical coupler 123 and output to the transmission path.
Furthermore, regarding the ring type optical transmission system as shown in FIG. 19, it is possible to exchange optical signals of various wavelengths between different ring networks by connecting a plurality of (in this case two) different ring networks to each other by a hub node in each network, as shown in FIG. 23 for example. FIG. 24 is an enlarged view of the connection between the hub nodes of the ring networks. As shown in FIG. 24, in each hub node, the WDM light is first demultiplexed into wavelengths λ1 to λn in the optical demultiplexer 101 which are then branched in two by the optical couplers 102. One of the optical signals is sent to the local ring network, and the other optical signal is sent to the adjacent ring network through a connecting optical path 105 provided between the rings. In FIG. 24, only the connecting optical path 105 corresponding to the wavelength λn is shown, but a similar connecting optical path is provided for the other wavelengths. Then whether the optical signals of the local ring network are multiplexed, or optical signals from the adjacent ring network are multiplexed in the optical multiplexer 104 is selected by an optical switch 103. By providing such a connection between hub nodes, optical signals of all wavelengths carried on the different ring networks can be exchanged between the various ring networks, and optical cross-connect can be realized.
However, in the aforementioned ring type optical transmission system as shown in FIG. 19, for example as shown in FIG. 25, when adding an optical signal with a given wavelength (in this case λ3, for example) to the transmission path from the central station N0 and transmitting the optical signal to the desired OADM node, the optical signal with the wavelength λ3 which comes from the central station N0 is terminated at the OADM node N3 which is allocated the same wavelength λ3 as the optical signal, and consequently, the optical signal with the wavelength λ3 cannot propagate to the OADM nodes N4 to N7 which are beyond the OADM node N3. In other words, conventional ring type optical transmission systems present a problem in that communication between the central station and an optional OADM node, as well as multicast communication and broadcast communication, are difficult to realize.
One method of solving such a problem is for example to set the wavelength of the optical signals transmitted from the central station N0 to a different wavelength from the wavelengths allocated to the OADM nodes. However, this presents a problem in that because a central station which has a need to communicate with all of the nodes on the ring network must be allocated the same number of wavelengths as the total number of wavelengths used in the OADM nodes, the number of wavelengths which can be used in the entire system is reduced by half. Furthermore, as shown for example in FIG. 26, it is possible to realize communication with an optional node, as well as multicast communication and the like, by making each OADM node of the ring network a hub node in the same manner as the central station. However, because a hub node must be compatible with every wavelength included in the WDM light, which increases the scale of the device, this presents a disadvantage in terms of size and cost.
Furthermore, regarding the system as shown in FIG. 23 in which a plurality of ring networks are connected to each other via hub nodes, because a rejection/add filter 121 (FIG. 22) is provided in each OADM node to prevent the same wavelength from recirculating, then when there is communication with an adjacent ring network via a hub node, as shown for example in FIG. 27, an optical signal with a wavelength λ3 added from an OADM node NA3 on a ring network A is terminated at an OADM node NB3 on a ring network B which is allocated the same wavelength λ3. Consequently, the optical signal of wavelength λ3 cannot propagate to the OADM nodes NB4 to NB7 which are beyond the OADM node NB3. That is to say, there is a problem in that multicast communication and broadcast communication between adjacent ring networks connected via hub nodes are difficult to realize.